In recent years, various kinds of flat panel display devices (“FPDs”) have been developed to replace the heavy and voluminous cathode ray tube. Such FPDs include, for example, liquid crystal displays (“LCDs”), field emission displays (“FEDs”), plasma display panels (“PDPs”), and electroluminescence displays (“ELDs”).
In particular, the PDPs are known to be the most advantageous in implementing a slim, light-weight and large-sized screen because its structure and fabrication process are simple. Nonetheless, the PDPs are also known to be disadvantageous in that they have low luminous efficiency and luminance, and large power consumption. Thus, the thin film transistor (TFT) LCDs instead have been most widely used but are disadvantageous in that they have a narrow viewing angle and a low response speed. The ELDs are largely classified into an inorganic light emitting diode display and an organic light emitting diode display according to materials used in an emission layer. Of the two, the organic light emitting diode display is a self-emitting element and is more advantageous in that it has faster response speed, higher luminous efficiency, and a larger viewing angle.
As shown in FIG. 1, the OLED, which is an organic electron element converting an electric energy into a light energy, has a structure in which organic emission materials for emitting light are placed between an anode electrode ANODE and a cathode electrode CATHODE. Holes are injected from the anode electrode, and electrons are injected from the cathode electrode. The holes and electrons are injected from the electrodes to an organic emission layer EML to form excitons. Specifically, the holes are recombined with the electrons in the organic emission layer EML, and the OLED emits light due to energy generated when the excitons returns to a bottom level. In order to smoothly inject the holes and electrons into the emission layer EML from the electrodes, typically, a hole transport layer HTL and a hole injection layer HIL are placed between the emission layer EML and the anode electrode. Further, an electron transport layer ETL and an electron injection layer EIL are placed between the emission layer EML and the cathode electrode.
For a smooth hole injection, the hole injection layer HIL and the hole transport layer HTL have an HOMO (highest occupied molecular orbital) level which corresponds to the middle level between the emission layer EML and the anode electrode. In addition, for a smooth electron injection, the electron transport layer ETL and the electron injection layer EIL have a LUMO (lowest unoccupied molecular orbital) level which corresponds to the middle level between the cathode electrode and the emission layer, EML. Brightness and efficiency characteristics of the OLED element are determined by the amount of the holes and electrons injected from the anode electrode and cathode electrode into the emission layer EML. The amount of the holes injected from the anode electrode into the emission layer EML and the amount of the electrons injected from the cathode into the emission layer EML are varied depending on an energy level of the organic emission material.
Meanwhile, in the OLED display, for implementation of full colors, the emission layer EML is formed at a position where the OLED is disposed in each of red, green, and blue pixels. The emission layer EML is patterned for each pixel. As methods of forming the emission layer EML, there have been known methods of using a fine metal mask (FMM), an ink jet method, a laser induced thermal imaging (LITI), or the like.
In particular, in the FMM method, red, green, and blue emission materials are patterned for each pixel using a metal fine mask to form red, green, and blue pixels. This method has superiority in terms of element characteristics. It, however, has a low yield due to the phenomenon of the mask blocking, and is hardly applied to a large-sized display device since a large-sized mask is difficult to develop.
The ink jet method is advantageous in that large-sized screen and high definition characteristics and high luminous efficiency can be implemented since the emission layer can be easily formed at selected regions and there is no damage to materials. In the ink jet method, however, there is a need of an accurate adjustment of the amount, the speed, the uniform jetting angle of ink jetted from nozzles. Also, there is a need of development of ink jet heads with a higher speed jetting and an increased number of heads for implementing lower cost and larger-sized screen. Furthermore, quality and thickness of the emission layer must be uniform so as to secure uniform emission in pixels. There, however, appears a so-called coffee stain effect where a periphery portion of the emission layer becomes thicker than a middle portion of the emission layer in a process of drying ink drops, and thus the periphery is thickened.
The laser induced thermal imaging (LITI) is a method in which a light source like a laser is irradiated to a transfer substrate including an organic emission material pattern, a light-to-heat conversion layer, and a support film to transfer the organic emission material pattern on the transfer film onto another substrate, thereby forming an emission layer. Describing this further in detail, in the laser induced thermal imaging, the transfer film provided with red, green, and blue organic emission material patterns is disposed on a substrate provided with black matrices, and thereafter the substrate and the transfer film are aligned and attached to each other. Next, the substrate to which the transfer film is attached is positioned on a stage of a laser irradiation device, and then the stage or a laser head moves from one end of the substrate to the other end thereof to perform a laser scanning. Thereby, a laser beam is sequentially irradiated to the red, green, and blue organic emission material patterns. Accordingly, the organic emission material patterns are sequentially transferred to the respective pixel regions on the substrate.
In the cases where the organic emission layers are formed on the substrate by the use of the laser induced thermal imaging in this way, a series of processes are repeated to form the red, green, and blue organic emission layers, where the respective transfer films corresponding to the red, green, and blue are attached to the substrate, the laser is irradiated thereto in the scanning manner, and then the transfer films are detached. Thus, the repeated fabrication processes cause the process time to be lengthened and the processes to be complicated. Further, there is a problem in that bad patterns are sometimes generated due to micro bubbles in the course of attaching and detaching the respective transfer films of red, green, and blue to the substrate. Also, there is another problem in that edges of the organic emission layers become rough by the repeated irradiation of the laser beam, and the attachment and detachment of the transfer films.
As discussed above, it is difficult to implement an organic emission layer having a high precision to a large-sized screen display using the FMM method, the ink jet or the LITI method.